The invention relates to a method for the manufacture of dense-sintered glass-ceramic moldings and glass-ceramic moldings manufactured according to said method, having a coefficient of thermal expansion .alpha., of -0.5.times.10.sup.-6 /K to 1.8.times.10.sup.-6 /K in the temperature range between 20 and 700.degree. C., whereby finely divided ceramizable glass and/or glass-ceramic powder from the system Li.sub.2 O - Al.sub.2 O.sub.3 -SiO.sub.2 in the slurry is molded by means of porous molds into green bodies, dried and then sintered and ceramized at temperatures of 700.degree. C. and above until reaching the sintering temperature up to relative densities of greater than 0.96. The invention also relates to the use of the shaped bodies manufactured according to the method.
When molten glass is cooled below the melting point of the crystals of identical composition, glasses are in the state of a supercooled liquid. The fact that the crystallization does not occur, is due primarily to the fact that the crystal growth, which is controlled by the diffusion of the components, proceeds much too slowly as a result of the viscosity of the molten glass, which increases rapidly with decreasing temperature, or the number of nuclei from which the crystallites (smallest particles in which the crystal structure can be identified), is too low. In glass-ceramics, on the other hand, the crystallite formation in suitable glass systems is stimulated to obtain materials which have certain characteristics.
The starting point for the manufacture of glass-ceramics is a molten glass from which first the desired objects are shaped, e.g. by pressing, blowing, rolling or casting. Submicroscopically fine crystallites are then formed during a subsequent heat treatment based on a precisely determined temperature-time curve. A prerequisite for the formation of such crystallites is the addition of substances which have a high melting point (generally TiO.sub.2 and ZrO.sub.2) to the molten glass, which particles, when they are precipitated as nucleating agents, initiate the crystallization. It is thereby essential that the temperature range of the maximum nucleation frequency is below the temperature range of the maximum crystal growth rate; in that case, the glass cannot crystallize during cooling, as long as there are no nuclei. Only when these nuclei have been formed in sufficient numbers in the temperature range of the maximum nucleation frequency can the desired tiny crystallites be obtained in a large number (up to 10.sup.17 /cm.sup.3) when reheated to the temperature of the maximum crystal growth rate. The crystal concentration in the volume can ultimately be 50-90%, depending on the desired characteristics of the finished product.
The importance of glass-ceramics for industrial applications is that their characteristics are determined not only by the vitreous ingredients, but also to a decisive extent by the types of crystals formed. In some particularly important systems, crystal phases are formed which have very low or even negative coefficients of thermal expansion (e.g. lithium-alumo-silicate). Using these systems, it is possible to create materials which experience almost zero expansion over a wide temperature range, retain their shape up to temperatures of approximately 800.degree. C. and are altogether insensitive to rapid changes in temperature. Such materials can therefore be used, for example, for cooktops, glass cooking utensils, telescope mirror mounts, measurement standards etc.
Hollow glass and glass for cookware and dinnerware can also be manufactured from glass-ceramics. The products from this group, which were first marketed under the trade name PYROFLAM.RTM. (Corning), opak and JENA 2000.RTM. (Schott) are transparent, and are also characterized by an coefficient of thermal expansion which is approximately zero. They can therefore be exposed to extremely rapid changes in temperature, e.g. they can be taken from a freezer and placed on a hot cooktop. The opaque glass-ceramics bear a visual resemblance to porcelain, which is the result of a coarser growth of the microcrystallites in the ceramization process.
While the molding of bowls, pans etc. previously used the press method exclusively, recent developments have made it possible to use the blowing method to shape glass- ceramic melts which are inherently difficult to process. It has thereby become possible to include thin-walled glass containers in the line of glass-ceramics products for domestic items, including decorated glass objects which are fully resistant to high temperatures and the thermal shocks caused by severe, sudden changes in temperature.
U.S. Pat. No. 3,600,204 discloses a crystalline ceramic object which has a coefficient of thermal expansion between 0-15.times.10.sup.-7 /.degree. C. in the range from 0.degree. C. to 1000.degree. C. and excellent dimensional stability at temperatures up to 900.degree. C., which consists of crystals which all have a diameter of less than 10 microns, and which are distributed in a vitreous matrix. The crystalline fraction thereby amounts to at least 90 wt. % of the object, and consists exclusively of .beta.-spodumene solid solution containing up to 15 wt. % mullite. The crystals are formed "in situ" from a glass composition (in wt. % on an oxide basis) which contains 3.5-7.5% Li.sub.2 O, 15-30% Al.sub.2 O.sub.3 and 65-80% SiO.sub.2, whereby the molar ratio of Al.sub.2 O.sub.3 :LiO.sub.2 is between 1.0 and 1.5, and the sum of Li.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2 makes up at least 98% of the total composition.
The object of U.S. Pat. No. 3,600,204 is to create a ceramic material which has excellent resistance to thermal shock and excellent dimensional stability, even under conditions of long-term use at temperatures up to 900.degree. C., primarily for heat exchangers.
U.S. Pat. No. 3,715,220 describes a ceramic object which consists of a mixture of sintered, solidified particles, up to 100 wt. % of which are less than 4 mesh. The object is characterized by a coefficient of thermal expansion of 0.+-.1.times.10.sup.-7 /.degree.C., in the temperature range between 5.degree. C. and 35.degree. C., by an open porosity of less than 1%, and by finely-divided inorganic crystals as the dominant components, distributed in a vitreous matrix.
The mixture of the particles thereby consists of two different glass materials:
45-50 wt. % of a first finely-divided glass material which contains (in wt. % on an oxide basis) 70-74% SiO.sub.2, 22-24% Al.sub.2 O.sub.3, 4-6% Li.sub.2 O and 0-2% other oxides, with a molar ratio of Al.sub.2 O.sub.3 :Li.sub.2 O of not more than 1.5, and PA1 45-55 wt. % of a second finely-divided glass material which contains (in wt. % on an oxide basis) 68-72% SiO.sub.2, 17-19% Al.sub.2 O.sub.3, 4-6% TiO.sub.2, 2-24% Li.sub.2 O, whereby the sum of SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2 and Li.sub.2 O makes up 95% of the glass composition of the second material, along with 2-4% MgO, 0-2% ZnO and 0-2% other oxides. PA1 heating from room temperature to approximately 1020.degree.-1080.degree. C. PA1 holding at 1020.degree.-1080.degree. C. for at least 30 minutes PA1 further heating from 1020.degree.-1080.degree. C. to 1150.degree.-1210 C. PA1 holding at 1150.degree.-1210.degree. C. for at least 30 minutes PA1 further heating from 1150.degree.-1210.degree. C. to 1250.degree.-1270.degree. C. PA1 holding at 1250.degree.-1270.degree. C. for at least one hour, followed by PA1 cooling to room temperature.
From this mixture of two different glass materials which are prepared together in a mill to form a grain size fraction which has a typical grain size distribution, a green molding is manufactured and subjected to the following thermal treatment:
The object of U.S. Pat. No. 3,715,220 is to use conventional shaping methods such as slurry casting, pressing, isostatic pressing, extrusion techniques, injection molding and similar processes to manufacture green moldings, and to carry out a thermal treatment so that dimensionally-stable materials are obtained which do not experience thermal expansion and can be used, for example, for cooktops, optical instruments, telescope mirror mounts and similar applications.